One common technique used in process control is to set the desired value of a process parameter, to monitor the actual value of that parameter as the process is taking place, to compare the actual value of the parameter with the set value thereof in order to derive an error signal, and to correct the actual value of the parameter depending on the error signal in order to make it approach the set value and stay equal to it. For example, if the parameter of interest is the rotational speed of a shaft, the actual speed of the shaft is monitored, the monitored actual speed is compared with a set value and, depending on the difference, if any, the device which rotates the shaft is speeded up or is slowed down.
Process controls, and specifically revolution speed controls, are classifiable as analog and digital. For analog control, an analog signal representing the actual value of a monitor parameter is derived and is compared with an analog signal representing the set value of that parameter. The difference is an analog signal (often called an error signal) which is applied to a control device in order to make the monitored value approach the set value. Analog process control is usually fast, but it is sometimes not sufficiently accurate. In digital process control, the actual monitored value of the parameter of interest is expressed in digital form and it is compared with a digital representation of the set value. The difference between the monitored and the set values is a digital error signal which is utilized by a digital control device. Digital control is generally more accurate than analog control, but it is somewhat slower, and is sometimes more expensive to implement.